A compact zoom lens having a high zoom ratio is requested for an image pickup optical system with increasing performance of cameras (digital cameras) using a solid-state image pickup element.
In order to decrease the size of the camera and increase the zoom ratio of the zoom lens, a so-called retractable zoom lens is widely used in which lens units are disposed closer to each other in a non-image pickup state than in an image pickup state to be retracted into a camera body.
Moreover, U.S. Pat. No. 6,333,823 and Japanese Patent Laid-Open No. 2004-102090 disclose bent-type zoom lenses in which an optical path thereof is bent by 90° using an optical element such as a prism including a reflective surface to reduce a thickness of a camera (hereinafter referred to as “camera thickness”).
Further, U.S. Pat. No. 7,426,085 discloses a zoom lens in which a lens unit disposed closer to an object than a reflective unit such as a prism is retracted into a space formed by movement of the reflective unit. Such a zoom lens, which corresponds to a combination of the retractable zoom lens and the bent-type zoom lens, is hereinafter referred to as “retractable bent-type zoom lens”.
Although the retractable bent-type zoom lens is capable of significantly decreasing the camera thickness and significantly increasing the zoom ratio, an optical system thereof should be designed with consideration for a mechanism enabling withdrawal of the reflective unit and movement (retraction) of the lens unit disposed closer to the object than the reflective unit (the lens unit is hereinafter referred to as “object-side lens unit”). However, an optical system disclosed in U.S. Pat. No. 7,426,085 in which a first lens unit disposed closest to the object is moved in a large movable range during zooming is disadvantageous in such a mechanism, so that the optical system does not has a realistic configuration for achieving decrease of the camera thickness and increase of the zoom ratio.
Specifically, in order to enable the withdrawal of the reflective unit and the retraction of the object-side lens unit, it is necessary to form a large opening on a cam barrel which moves the object-side lens unit. Forming such a large opening on the cam barrel reduces a rotatable angle of the cam barrel, which results in reduction of a movable range of the first lens unit to a range enabling only movement thereof from an image pickup state to a retracted state. In order to enable movement of the first lens unit during zooming, as disclosed in U.S. Pat. No. 7,426,085, it is necessary to compromise at least one of requirements of a size (thickness), the zoom ratio and image-forming performance of the zoom lens.